Offshore wind energy is one of the renewable energy sources to be exploited in Europe, Asia and North America to reduce anthropogenic emission of greenhouse gases from energy mix. Accurately estimating the wind-waves and temperature climate is important to the ocean and coastal engineering design. The Global Change Observation Mission-Water "Shizuku" (GCOM-W) satellite, with a newly developed microwave radiometer: Advanced Microwave Scanning Radiometer-2 (AMSR2) developed by Japan Aerospace Exploration Agency, was launched successfully in May 2012. The standard geophysical products of AMSR2/GCOM-W were released a year after launch date. Here, we use data from three buoys moored in the North Pacific Ocean to test the accuracy of AMSR2 sea surface temperature (SST) and near surface wind speed (SSW). The North Pacific Ocean is subject to large multi-scale variability and intense air-sea interaction and thus provides a challenging test for the satellite sensor. From the year-long comparison, we confirm that the root mean square difference (RMSD) of AMSR2 SST observations was 0.75°C and meets the criterion for release accuracy (0.8°C). On the other hand, the RMSD of SSW was 1.6 m s-1, slightly worse than the criterion (1.5 m s-1), suggesting that the algorithm for SSW needs to be further improved. The analysis also showed that seasonal variations and characteristics of the relationship between SST and SSW are similar to those observed by previous satellite sensor (AMSR-E). Overall, the results give confidence that AMSR2 products can be used for many air-sea interaction, climate, and water cycle studies.

Method

The region of Japan's Seas, which includes Kuroshio Extension area in the North Pacific Ocean, are often threatened by extreme events like typhoons and storm waves. Wind sensors were mounted on the buoy's tower at a height of 3.5-4.5 m from sea surface, AMSR2 standard geophysical products gives wind speed at 10 m. There are two kinds of data for SSW and SST: L2 and L3. In the match-up process, we found roughly ten outliers only for SSW, with very large differences ( > 10 m s-1). For SST, the release, standard, and goal threshold accuracies are 1.0°C, 0.5°C and 0.2°C, respectively, while those of the SSW are 1.5 m s-1, 1.0 m s-1 and 1.0 m s-1.

Results

AMSR2 SSW is biased high (+1.04 m s-1) relative to the buoy winds (10 m height). Larger differences were found in the low (< 5m s-1) and high (> 15 m s-1) wind range. The large discrepancies in high wind range were mainly observed by KEO and are associated with typhoons. The monthly climatologies for both data sets agree to within their SD. In the comparison with buoy, there was difference between ascending and descending observations for SSW: ascending have larger RMSD (1.70 m s-1),descending (1.46 m s-1). Difference of 0.24 m s-1 is significant as confirmed by F-test. For ∆T derived from AMSR2 and that from buoys, the average over the entire period is nearly zero.

Conclusions

The newly developed satellite microwave sensor AMSR2 mounted on the GCOM-W satellite, launched in May 2012, is providing many standard geophysical parameters, including sea surface temperature and sea surface wind. This study shows that based upon in situ observations in the KE region, SST meets the accuracy criterion for public release. SSW does not meet the criterion; the RMSD is slightly worse (1.6 m s-1) than the criterion (1.5 m s-1). The fact that AMSR2 can observe SST and SSW near the release accuracy threshold in this challenging region is very encouraging. The accuracy, however, does not meet the "goal" criterion. It is hoped that the current algorithm can be improved to better capture the range of variability of both SST and SSW.

Objectives

This research found larger discrepancies relative to the buoy observations when complex fronts and eddies were in the region of the buoy. AMSR2 spatial resolution for SST is approximately 50 km. In regions where very sharp fronts can persist for long periods, this can produce local biases in the satellite fields. For roughly 77% of the time record, winds are greater than 5 m s-1, which suggests that strong diurnal warming by solar radiation is not expected to produce afternoon thermal stratification. The accuracy, however, does not meet the "goal" criterion. It is hoped that the current algorithm can be improved to better capture the range of variability of both SST and SSW.